This invention relates to radiation detecting devices and, more particularly, to low capacitance diode detectors for converting optical radiation into electrical signals.
This invention is an improvement over the invention described and claimed in Baertsch et al. U.S. Pat. No. 4,146,904 issued Mar. 27, 1979 and assigned to the present assignee.
In computerized tomography systems, X-ray radiation detectors are employed to produce electronic signals in response to impingment of X-rays thereon. From these signals, viewable images of internal portions of a subject through which the X-rays pass can be constructed. To create these signals, the X-ray detectors in computerized tomography systems employ scintillators to convert the X-ray radiation into light that can be detected by photodetectors from which the electronic signals are produced. A scintillator, however, imposes limits on how fast the detector can operate and, in order to exceed these limits, scintillators of low efficiency in converting X-rays into optical radiation have been employed. Such scintillators produce lower output light intensity for any given level of X-ray intensity than comparable scintillators of higher efficiency.
In order to obtain benefit of the high speed but reduced output light intensity of a low efficiency scintillator in an X-ray radiation detector, a more sensitive photodetector than normally used for a high efficiency scintillator must be employed. This, in turn, requires that the photodetector have a high signal-to-noise ratio. The present invention is therefore directed to an x-ray radiation detector employing a photodetector of high sensitivity to optical radiation.
Semiconductor diode detectors are used in various radiation sensing applications. Spurious signals, known as "noise", however, tend to degrade the information sensed by such detector. The primary constituents of the noise in systems employing such detectors are amplifier noise, which is a function of detector capacitance, and dark current shot noise. Reduced noise levels lead directly to sensitivity improvements which are highly desirable in many applications.
A low capacitance semiconductor diode can be constructed by minimizing the junction area and employing doping gradients to prevent minority charge carriers from reaching the front surface (i.e., radiation-receiving surface) where they can be lost through surface recombination, and by using either a thin wafer with a back surface doping gradient, or an epitaxial layer on a heavily doped substrate to prevent the minority charge carriers from being lost through back surface recombination or from diffusing deep into the semiconductor where they can be lost through bulk recombination. A primary requirement is that the minority charge carrier recombination diffusion length be long or at least comparable to the junction collector spacing and active layer thickness.
Reduction in junction area also leads to reduction in the volume of depleted semiconductor, which in turn leads to reduced dark current or, in the case of operation at zero bias, an increase in diode shunt resistance. Diode thermal noise is reduced in either case.
The state-of-the-art in detector diodes is the PIN detector diode, a P type/Intrinsic layer/N type structure in which diode capacitance is determined by the thickness and purity of the intrinsic layer. The diode of the present invention is constructed with doping levels similar to those of the PIN detector diode, but with the junction area reduced to reduce both capacitance and dark current. Collection efficiency is maintained by using doping gradients to prevent the generated charge from reaching surface and bulk recombination regions, thereby increasing the probability of collection by the reduced area junction.